Leadframe Strip And Leadframes

ABSTRACT

A leadframe strip including a first leadframe having a first die pad and a first plurality of generally parallel leads each extending outwardly relative to the first die pad and terminating in a free end and a second leadframe having a second die pad and a second plurality of generally parallel leads extending outwardly relative to the second die pad and terminating in a free end. The free ends of the second plurality of leads are positioned in close nontouching adjacent relationship with the free ends of the first plurality of leads. The two leadframes may be separated from each other by a single saw cut.

BACKGROUND

Integrated circuit packages typically comprise at least one leadframeand at least one integrated circuit die. The integrated circuit die isgenerally mounted on a die pad of the leadframe and contacts on the dieare usually electrically connected to leads of the leadframe. Theseleads may also be electrically connected to other circuitry inside oroutside the integrated circuit package. The die and portions of theleadframe are usually encapsulated in an epoxy mold compound. Sinceintegrated circuit packages are used for many different purposes, thereare many different types of integrated circuit packages.

Most leadframes are formed on a single, thin sheet of metal as bystamping or etching. Multiple interconnected leadframes may be formed ona single leadframe sheet. Leadframes on the sheet are usually arrangedin rows and columns. Usually die mounting, die to lead attachment (e.g.wire bonding) and epoxy encapsulation take place while the leadframesare still integrally connected on the leadframe sheet. After suchprocesses are completed, the leadframe, and sometimes the encapsulationmaterial, are severed (“singulated” or “diced”) with a cutting tool,such as a saw or laser. These singulation cuts separate the leadframesheet into separate integrated circuit packages, each typicallyincluding at least one leadframe, at least one die, electricalconnections between the die and leadframe (such as gold or copper bondwires) and the plastic material which encapsulates at least part ofthese structures.

One common type of integrated circuit packages is “SOP” (Small OutlinePackages) in which leads protrude from opposite sides of theencapsulated portion of the package. Another common type is known as“QFP” (Quad Flat Packages). In QFP, the leads protrude outwardly fromfour lateral sides of the encapsulated portion of the package. Theprotruding leads in, for example a Gull Wing type SOP, may be bent intoan appropriate shape after the integrated circuit package has beensevered from the leadframe. Such integrated circuit packages are oftenused in surface mount technology (“SMT”) in which the leads extendingfrom the sides of the package are soldered to surface contact portionsof a printed circuit board or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a prior art leadframe strip.

FIG. 2 is a top plan view of a portion of a prior art leadframe strip,such as shown in FIG. 1, after die mounting, wire bonding, andencapsulation (molding).

FIG. 3 is a top plan view of a portion of a prior art leadframe stripsuch as shown in FIG. 1, having dies mounted on leadframe portionsthereof.

FIG. 4 is a schematic top plan view of portions of a prior art diemounted, wire bond connected and molded SOP leadframe assembly includinga plurality of outwardly extending leads that are connected by first andsecond dam bars.

FIG. 5 is a top plan view of a prior art assembly with two adjacentleadframe assemblies such as shown in FIG. 4, wherein leads of eachleadframe are connected by first and second dam bars.

FIG. 6 is a schematic isometric view of a prior art assembly including aleadframe lead with an unpiated terminal end engaging a contact portionof a printed circuit board.

FIG. 7 is a schematic isometric view of the lead and printed circuitboard contact portion of FIG. 6 after solder bonding.

FIG. 8 is a top plan view of a portion of an example embodiment of a newSOP type leadframe strip in which each leadframe portion thereof hasleads connected by first dam bars and in which there are no second dambars.

FIG. 9 is a top plan view of portions of a die mounted, wire bondconnected and encapsulated SOP leadframe strip assembly including twoleadframes of the type illustrated in FIG. 8,

FIG. 10 is a schematic top plan view of a portion of an assembly such asshown in FIG. 9 and showing a singulation saw path.

FIG. 11 is a schematic top plan view of a portion of an SOP type ICpackage that includes one of the encapsulated leadframes of FIGS. 9 and10 after singulation.

FIG. 12 is a top plan view of an SOP type leadframe strip, a portion ofwhich is shown in FIG. 8.

FIG. 13 is an isometric view of part of a leadframe lead having a platedtip portion, with the lead engaged with a contact of a printed circuitboard.

FIG. 14 is a top plan view of the assembly of FIG. 13 with a layer ofsolder bonding the lead to the printed circuit board contact.

FIG. 15 is a flow chart of a method of making a plurality of integratedcircuit packages.

DETAILED DESCRIPTION

This description, in general, discloses a leadframe strip 110, FIGS. 8and 12, that comprises a first leadframe 112 and a second leadframe 114,which are integrally connected. The first leadframe 112 has a die pad122 and a first plurality of generally parallel leads 130, e.g., 132,134, each extending outwardly relative to the die pad 122 andterminating in free ends, e.g., 133, 135, etc. The leadframe strip 110includes a second leadframe 114 having a second die pad 122A and asecond plurality of generally parallel leads 140A, for example, 142A and144A, extending outwardly relative to the second die pad 122A andterminating in free ends, e.g., 143A, 145A. The free ends 143A, 145A,etc., of the second plurality of leads 140A are positioned innontouching adjacent relationship with the free ends 133, 135, etc., ofthe first plurality of leads 130.

Having thus generally described an embodiment of a new leadframe strip,certain prior art leadframe embodiments and methods of use thereof willnow be described. The new leadframe embodiments and methods of usethereof for making integrated circuit (“IC”) packages are then describedin detail.

FIG. 1 is a top plan view of a prior art leadframe strip 10. This strip10 has a leadframe area 11 that includes a gridwork of interconnectedleadframes 12, 14, 16, 18, etc., which are arranged in rows 22, 24, etc.and columns 26, 28, etc. Side rails 32, 34 that facilitate handling ofthe leadframe area 11 are integrally attached at opposite sides of theleadframe area 11. Connecting strips 36, 38, etc, are positioned betweenadjacent columns, e.g., 26, 28, etc. of lead frames.

FIG. 2 is a top plan view of a portion of a prior art leadframe sheet10. It includes three interconnected leadframes 40A, 40B, 40C that areheld in fixed relationship with one another by leadframe connectingstrips 36, 38. Each of the leadframes has a die pad 42, which issometimes referred to in the art as a “die attachment pad” or “diepaddle.” The die pad 42 is initially connected to other portions of theleadframe, e.g. 40A as by thin metal bars (not shown in FIG. 2), whichare removed during IC package formation. A plurality of longitudinallyextending leads 44, 46, 48, etc., extend outwardly relative to a firstlateral side of the die pad 42. A second plurality of longitudinallyextending leads 43, 45, 47 extend outwardly relative to a second lateralside of the die pad 42. A first (inner) damn bar 52 is integrally formedwith and intersects portions of each of the leads 44, 46, 48, etc., nearthe proximal end of those leads, i.e., at a location close to the die42. A second (outer) dam bar 54 is integrally formed with and connectsdistal (outer) end portions of each of the leads 44, 46, 48. Similarlylongitudinally leads 43, 45, 47, etc., extend outwardly relative to theother side of the die pad 42. A first (inner) damn bar 53 intersectsportions of leads 43, 45, etc. near the proximal ends thereof. A second(outer) dam bar 55 connects the distal end portions of leads 43, 45,etc.

FIG. 3 is a top plan view of a portion of a leadframe strip of FIG. 2after an integrated circuit chips (not visible) has been attached toeach of the die pads 42, FIG. 2), inner portions of each of the leadse.g., 44, 46, 48, etc., have been conventionally wire bonded to acorresponding electrical contact surface on the die; and 3) the die (notshown and bond wires (not shown) and an inner portion of each of theleads, e.g., 44, 46, 48, etc. has been encapsulated in epoxy moldcompound 62.

FIG. 4 is a schematic representation showing leads 42, 44, 46, etc. of asingle prior art leadframe, such as 40A or 40B in FIG. 3, beforesingulation. It also shows the connections of the first dam bar 52 andsecond dam bar 54 to the associated leads 42, 44, 46, etc. This entirecombination of leads 42, 44, etc., and first and second dam bars 52, 54is sometimes referred to in the art as the “outer leads.”

FIG. 5 is a top plan view similar to that of FIG. 2. It shows the leads43, 45, 47, etc., of leadframe 40A connected by a first damn bar 51 atthe proximal ends thereof and second dam bar 53 at the distal endsthereof. It also shows that the leads 44, 46, 48, etc. of die 40B arealigned with corresponding leads 43, 45, 47, etc., of the leadframe 40A.Leads 44, 46, 48, etc., are connected by first 52 and second dam bars52, 54 of leadframe 40B. FIG. 5 also illustrates the cut paths or sawstreets AA and BB that are used to remove the second dam bars 53, 54 andend portions of each of the leads during singulation of the die mounted,wire bonded and molded leadframes 40A 40B. It will be appreciated fromFIG. 5 that a substantial portion of the leadframe strip 10, FIG. 1, istaken up by the second dam bars 53, 54 and associated end portions ofattached leads, i.e., the entire area between AA and BB, which are laterremoved. The portions of each inner dam bar 51, 52 that extend betweenadjacent leads, e.g., 44, 46, are removed during package formation.

The specific type of leadframe illustrated in the embodiments of FIGS.1-5 is an SOP type leadframe in which the leads are designed to projectoutwardly from two opposite sides of the SOP produced using this typeleadframe. It will be appreciated that a QFP type leadframe hasoutwardly projecting leads on each of its four lateral sides that extendoutwardly from the molded portion of the package. Each lateral side of aQFP package has first and second dam bars connecting the leads, whichare subsequently removed during package formation.

FIG. 6 illustrates a prior art lead 70 extending from an associatedintegrated circuit package, which was formed from a leadframe, such asmolded leadframe 40A in FIG. 5, and which has the dam bars trimmed formthe lead 70. The lead 70 and associated leadframe is made from a firstsubstance, such as copper, which is subject to oxidation/corrosion. Thelead 70 is plated with a plating layer 72 made from a substance, such asfor example nickel, palladium, gold or tin, that is less subject tocorrosion than copper. This plating process takes place before thesawing/singulation step shown in dashed lines in FIG. 5. In one priorart embodiment the entire leadframe is plated before the moldingprocess. In another prior art embodiment, just the exposed portions ofthe leads are plated, after molding but before singulation. With eitherplating method, singulation exposes copper at tip 74 of the lead 70. Thelead 70 in this embodiment is a gull wing type lead that has beenappropriately bent for attachment to a contact portion 80 of a printedcircuit board. In other prior art IC packages the leads are not bentprior to attachment.

As shown in FIG. 7, the lead 70 is attached to printed circuit boardcontact 80 with a layer of solder 86. In some cases the solder 86 is notplaced around the area of the tip 74, in other cases (as shown) it is.However, because the copper at tip 74 is exposed, even when solder isplaced in contact with the tip 74, the exposed tip 74 does not wet welland thus the solder bond between tip 74 and contact 80 is usually poor.Thus, the exposed copper tip 74 may render the lead 70 subject todisconnection from contact portion 80 of the printed circuit board 80.

FIG. 8 is a top plan view of a portion of a new type of leadframe strip110. The full leadframe strip 110 is shown in FIG. 11. As shown by FIG.8, included in the leadframe strip 110 is a first leadframe 112 andidentical second and third leadframes 114, 116 that are connected byparallel connecting strips 178, 180, which may be the similar to theconnecting strips 36, 38 described with reference to FIG. 1. The firstleadframe 112 has a first die attach pad 122. The die attach pad 122 hasa first laterally extending side 124 and an opposite second laterallyextending side 125. The die attach pad is initially attached to the restof the leadframe as by support strips (not shown), which areconventionally removed during package formation.

As shown by FIG. 8, leadframe 112 has a first plurality of leads 130,including leads 132, 134, 136, etc., extending longitudinally outwardlyrelative the first laterally extending side 124 of the die attach pad122. Each of these leads 132, 134, 136, etc., has a proximal end, forexample 139 of lead 136, positioned in close, spaced apart relationshipwith the lateral side, e.g., 124 of the die attach pad 122. Theseproximal ends are initially connected to one another by a first damn bar137 when the leadframe is fabricated. This first dam bar 137 issubsequently removed from between adjacent leads in a conventionalmanner at the time of singulation, thereby separating the leads 130 suchthat they are not in electrical contact with one another. Each of theleads 132, 134, etc., has a corresponding distal end 133, 135, etc. Thedistal ends 133, 135, etc., are free ends, i.e., these distal ends 133,135, etc., are not connected to one another by a damn bar. (All of theleadframes 112, 114 and 116 have leads formed without second (outer) dambars so as to provide such free ends.) A second plurality of leads 140extend longitudinally outwardly from positions near the second lateralside 125 of the die attach pad 122. The second plurality of leads 140may be the mirror image of the first plurality of leads 130. The secondplurality of leads 140 includes leads 142, 144, etc. havingcorresponding free ends 143, and 145, etc., and having correspondingproximal ends connected by a first damn bar 146.

As previously mentioned the leadframes 112, 114, 116 in the illustratedembodiment of FIG. 8 are identical. Corresponding portions of Ieadframes114 and 116 are labeled identically to leadframe 112, except thatreference numerals for leadframe 114 end with an “A” and for leadframe116 end with a “B”. The leadframes 112, 114, 116 are constructed andarranged such that the free ends of the first set of leads, e.g., 130 ofone leadframe, e.g. 112 are positioned in nontouching, close, alignedrelationship with those of the second set of leads, e.g. 140A of anadjacent leadframe, e.g., 114.

FIG. 9 is a top plan view of two leadframe portions 112 and 114 of thenew type leadframe strip 110 of FIG. 8, after completion of diemounting, wire bonding and molding. With respect to leadframe portion112, a layer of encapsulating epoxy 121 covers a first die 120, a firstdie attach pad 122 positioned below the die 120, inner portions of leads130 and 140 and bond wires 127 that connect the inner portions of theleads to contact pads on the die 120, as described in further detailbelow.

A first die 120 (shown only partially in FIG. 9) is mounted on the dieattach pad 122 of the leadframe 112. The die attach pad 122 has a firstlaterally extending side 124 and an opposite second laterally extendingside 125 (not visible in FIG. 9). A plurality of leads 130, includingleads 132, 134, etc., extend longitudinally outwardly from a positionproximate the first laterally extending side 124 of the die attach pad122. Bond wires, e.g. 127 connect contact surfaces, e.g., 129 on the die120 to an inner portion, e.g. 125, of a corresponding lead, e.g., 136.As previously mentioned, the proximal ends of the leads are initiallyconnected to one another by a first damn bar 137 when the leadframe isfabricated. this first dam bar 137 is subsequently removed in aconventional manner after a molding step, thereby separating the leadssuch that they are not in electrical contact with one another. Each ofthe leads 132, 134, etc., has a corresponding distal end 133, 135, etc.The distal ends 133, 135, etc., are free ends, i.e., these ends 133,135, etc., are not connected to one another by a damn bar. A secondplurality of leads 140 from the first leadframe 112 also extendlongitudinally from near the second lateral side of the die attach pad122. The second plurality of leads 140 may be the mirror image of thefirst plurality of leads 130. As previously mention with reference toFIG. 9, the first set of leads 130 of the leadframe 112 are positionedin close aligned relationship with the second set of leads 140A ofleadframe 114.

FIG. 10 is a schematic top view of adjacent portions of the encapsulatedleadframes 112 and 114 shown in FIG. 9. As shown by FIG. 10, the tipportions 133, 135, etc. of the first leadframe 112 are positioneddirectly opposite to the tip portions 143A, 145A of the second leadframe114. Each corresponding set of aligned lead tips, e.g., 133, 143A; 135,145A, are separated by a small gap, e.g. 152, 154. A singulation cuttermakes a single cut XX that extends through these gaps 152, 154, etc. toseparate the leadframes 112, 114.

FIG. 11 is a schematic top plan view of an integrated circuit package158, which includes leadframe 112. It is shown after it has beensingulated from the rest of the die mounted, wire bonded and moldedleadframe strip shown in FIGS. 9 and 10. At this point the first dam bar146 has been conventionally removed from between adjacent leads, asindicated by dashed lines in FIG. 11.

FIG. 12 illustrates a leadframe strip 110 in which the individualleadframes have the structure shown in FIGS. 8 and 9. Like the prior artleadframe strip 10 of FIG. 1, the new leadframe strip 110 has aleadframe grid portion 162 bordered on each side by a rail 117, 118. Theleadframes that are singulated from the new leadframe strip 110 are thesame size as those in the prior art strip 10. The number of leadframesin the new leadframe strip 110 is the same as the number of leadframesin the prior art strip 10. However, by eliminating the second damn barsfrom the structure of each leadframe, the leadframe grid portion 162 ofthe new strip 110 may have a reduced dimension between side rails 164,166. This dimension may be on the order of 10% to 15% smaller than thatof the corresponding dimension of the leadframe strip 10. The length ofeach leadframe strip 10 and 110 is the same, thus the total area betweenside rails of the new strip 110 is about 10% to 15% smaller that of thecorresponding area of the old strip. Since the cost of a leadframe stripis roughly proportional to the size of the area between rails, the newstrip 110 may achieve a savings in material cost on the order of 10% to15% over the prior art strip 10.

FIGS. 13 and 14 illustrate another advantage of the new leadframe strip110 over the prior art. FIG. 13 illustrates a lead 144 of an integratedcircuit die package, such as integrated circuit package 158, illustratedin FIG. 11. The lead 144 has been bent into an appropriate shape forattachment to a contact surface 181 of a printed circuit board. Theouter surface of the lead 144 in the portion of the lead that is to beattached to the contact surface 181 has a metal plating layer 141 thatcompletely covers the lead 144, including the tip portion 145. The tipportion 145 in FIG. 14 is plated because the lead 144 was not severedafter it was plated. Whether the plating process takes place before orafter the leadframe strip 110 is molded, there is no reason to sever thelead 144 after plating because it is not connected to adjacent leads bya dam bar. Also, at the time the leadframe strip 162 is made, all of theleads are made the correct length for their intended purpose so that nosubsequent severing of the leads is needed.

As shown by FIG. 14, since the tip 145 of lead 144 is plated, the solder190 bonds to the plated tip 145 as well as to the other plated portionsof the lead 144. For that reason the lead 144 with the plated tip 145will generally provide an improved connection of the lead 144 to theprinted circuit board, as compared to that of the prior art shown inFIG. 7.

FIG. 15 discloses a method of making a plurality of integrated circuitpackages. The method includes, as shown at 221, providing a metal stripand, as shown at 222, on the metal strip, forming a first leadframehaving a first die pad, a first plurality of leads with proximal endsnear to the first die pad and free distal ends positioned outwardly fromthe first die pad and a dam bar intersecting the proximal ends of thefirst plurality of leads. The method further includes, as shown at 223,on the metal strip, forming a second leadframe having a second die pad,a second plurality of leads with proximal ends connected to the seconddie pad and free distal ends positioned outwardly from the second diepad and a dam bar intersecting the proximal ends of the second pluralityof leads.

Although a detailed description of certain embodiments of a leadframestrip and leadframes provided thereby and methods of making integratedcircuit packages are described in detail herein, alternative embodimentsof such leadframe strips, leadframes and methods will become obvious tothose skilled in the art, after reading this disclosure. For example,while the illustrative embodiments of leadframe strips shown herein areall SOP type leadframes, a similar improvement in leadframe strip costand lead bonding to PC boards, etc., may be achieved for QFP typeleadframes by making QFP type leadframe strips with outwardly extendingleads that terminate in free distal ends on all four sides of the diepad. It is intended that the appended claims be broadly construed so asto cover such alternative embodiments, except as limited by the priorart.

1. A leadframe strip comprising: a first leadframe having: a first dieattach pad having opposite first and second lateral sides and oppositefirst and second longitudinal sides; a first plurality of leadsextending longitudinally outwardly relative to said first lateral side,each of said first plurality of leads having a free distal end and aproximal end; a laterally extending dam bar having a plurality of firstdam bar portions connecting adjacent ones of said first plurality ofleads at said proximal ends thereof; a second leadframe having: a seconddie attach pad having opposite first and second lateral sides andopposite first and second longitudinal sides; a second plurality ofleads extending outwardly relative to said second lateral side of saidsecond die attach pad, each of said second plurality of leads having afree distal end and a proximal end; and a laterally extending dam barhaving a plurality of dam bar portions connecting adjacent ones of saidsecond plurality of leads at said proximal ends thereof; wherein saidfree ends of said second plurality of leads are positioned in alignedadjacent nontouching relationship with corresponding free ends of saidfirst plurality of leads.
 2. The leadframe strip of claim 1 furthercomprising: a first longitudinally extending strip of material connectedto said first longitudinal side of said first leadframe and said firstlongitudinal side of said second leadframe; and a second longitudinallyextending strip of material connected to said second longitudinal sideof said first leadframe and said second longitudinal side of said secondleadframe.
 3. The leadframe strip of claim 1 wherein said free distalends of said first plurality of leads are entirely plated with amaterial different than the material form which said plurality of leadsare constructed.
 4. A method of making a plurality of integrated circuitpackages comprising: providing a metal strip; on the metal strip,forming a first leadframe having a first die pad, a first plurality ofleads with proximal ends adjacent to the first die pad and free distalends positioned outwardly from the first die pad and a first leadframedam bar intersecting the proximal ends of the first plurality of leads;and on the metal strip, forming a second leadframe having a second diepad, a second plurality of leads with proximal ends adjacent to thesecond die pad and free distal ends positioned outwardly from the seconddie pad and a second leadframe dam bar intersecting the proximal ends ofthe second plurality of leads, said free distal ends of said secondplurality of leads being aligned with and adjacent to said free distalends of said first plurality of leads.
 5. The method of claim 4 furthercomprising mounting a first die on the first die pad.
 6. The method ofclaim 5 further comprising wire bond connecting contact portions of thefirst die to the proximal ends of the first plurality of leads.
 7. Themethod of claim 6 further comprising encapsulating the first die and theproximal ends of the first plurality of leads and the wire bonds withmold compound.
 8. The method of claim 7 further comprising plating atleast the free distal end portions including the tips of the firstplurality of leads with a material different from the material fromwhich the first leadframe is made.
 9. The method of claim 8 wherein saidplating comprises plating prior to said mounting the first die on thefirst die pad.
 10. The method of claim 8 wherein said plating comprisesplating after said encapsulating.
 11. The method of claim 4 furthercomprising separating the first leadframe from the second leadframe witha cut extending between the free distal ends of the first plurality ofleads and the free distal ends of the second plurality of leads.
 12. Themethod of claim 4 further comprising removing portions of the first dambar between adjacent ones of the first plurality of leads.
 13. Themethod of claim 7 comprising removing portions of the first dam barbetween adjacent ones of the first plurality of leads after saidencapsulating.
 14. The method of claim 8 further comprising soldering atleast one of the leads on the first leadframe to a circuit boardincluding soldering the plated tip portion of the at least one of theleads to the circuit board.
 15. A leadframe strip comprising: a firstleadframe having a first die pad and a first plurality of generallyparallel leads each extending outwardly relative to said first die padand terminating in free ends; and a second leadframe having a second diepad and a second plurality of generally parallel leads extendingoutwardly relative to said second die pad and terminating in free endswith said free ends of said second plurality of leads positioned innontouching adjacent relationship with said free ends of said firstplurality of leads.
 16. The leadframe strip of claim 15 wherein saidfirst plurality of leads comprise proximal ends located adjacent to saidfirst die pad and said first leadframe further comprises a dam barintersecting said proximal ends.
 17. The leadframe strip of claim 15further comprising first and second parallel strips extending in thedirection of said first plurality of parallel leads that are attached tosaid first and second die pads on opposite sides of said first andsecond die pads.
 18. The leadframe strip of claim 15 wherein said firstleadframe is constructed from a first substance and wherein at least tipportions of said free ends of said first plurality of leads are platedwith a material different from said first substance.
 19. The leadframestrip of claim 18 wherein said first plurality of leads compriseproximal ends positioned proximate to said first die pad and said firstleadframe further comprises a dam bar intersecting said proximal ends.20. The leadframe strip of claim 18 further comprising first and secondparallel strips extending in the direction of said first plurality ofparallel leads that are attached to said dam bars on opposite sidesthereof.